In machines for treating and processing flowing, liquid and/or gaseous media, gaps between movable and stationary components are often to be sealed off from the flowing medium. This also applies in particular to turbines to which steam is admitted, in which a gap is sealed off between a rotor and a casing surrounding the latter in order to block the path of the steam past blade rings. The quality of these seals has a considerable effect on the efficiency of these machines, thus in particular also in the case of steam turbines.
Sealing strips arranged axially one behind the other—also called labyrinth seals—are normally used for this purpose in steam turbine construction. These seals are characterized by sealing strips which are arranged transversely to the flow and which virtually completely close a gap which is usually several millimeters wide. In this case, it is accepted that the sealing strips sometimes graze the component opposite them during transient processes and become slightly worn themselves at the same time. Such labyrinth seals are used in turbine construction both at blading and as piston and shaft seals.
A special form of these seals which has the same effect is a honeycomb seal. This seal, on one side of the gap, usually on the side fixed to the casing, has a structure which reproduces a honeycomb and on whose open surface a leakage flow is prevented by a multiplicity of small vortices in chambers formed by the honeycomb structure. A flow resistance produced as a result prevents a free flow in the passage defined by the honeycomb-like structure on one side.
U.S. Pat. No. 4,177,004 discloses a turbine in which a gap between a turbine blade and a ring enclosing the latter in the circumferential direction and suspended in a casing is to be sealed off. This arrangement is designed in such a way that the turbine blade itself occasionally grazes the ring enclosing it. In order to avoid impending damage in this case, the ring is coated with a material which causes no wear on the turbine blade.
However, both in the known labyrinth seals and in the arrangement according to U.S. Pat. No. 4,177,004, contact between the surfaces of the components moving along one another occurs only very rarely. This is because the components involved are at such a large distance from one another that contact actually takes place only occasionally during extremely transient states. On the other hand, the result of this is that—apart from the rare moments of the contact between the components—there is a gap through which a proportion of a working medium, which proportion is not to be disregarded, flows past the turbine blade without being utilized.